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Plant-to-Plant Translocation ofmRNA

Growing evidence indicates that mRNA is trafficked over long distances

in plants through the phloem and that this process plays an important role in

regulating plant development. Messenger RNA has even been found to move

across grafts between separa te species. This raises the question of whethermRNA is also translocated from host plants into plants that naturally parasitize them?

Dodders are parasitic plants that obtain resources by drawing from

the phloem (and xylem) of a host plant. They even form joint plasmodesmatal

connections with host cortical cells. Although viruses are known to movebetween dodder

and its hosts, translocation of endogenous plant mRNA has not been reported. In this

issue, Roney et al. (pp.1037-1043) show thatphloem- mobile mRNAs are translocated

from a host tomato (Lycopersicon esculentum) plant to the parasite lespedeza dodder

(Cuscuta pentagona; Fig. 1). Reverse transcriptase-PCRand microarray analysis revealed

the presence offourtoma to transcripts in doddergrown on tomato that were notpresent

in control dodder grown on otherhost species. These results point to a potentially new

level of in terspecies comm unication, and raise questions about the ability ofparasites to

recognize, use, and respond to transcripts acquired from their hosts.

Figure 1. Theparasiticplant (Cuscutapenta-gana) forms connections to the phloem andxylem 01 hostplants. Symplastically trans-ported host mRNAs canbe isolated from theparasiticplant. Photoby Charles T. Bryson(U.S. Department ofAgriculture).

Effects of lncreased NADP(H) Reductase on Photosynthesis

With the aim of genetically engineering enhanced productivity in plants, considerable

effort has been aimed at identifying the biochemical steps that limit photosynthesis.

Relatively few in- vestigations, however, have been carried out with genotypes modified

in their photosynthetic electron trans- port chain components. Ferredoxin- NADP(H)

reductase (FNR) catalyzes the last step of photosynthetic electron transport in

chloroplasts, transferring electrons from red uced ferredoxin to NADP+. Previous

analyses of plants transformed with an antisense version of FNR supported the idea that

FNR mediates a rate-limiting step of photo- synthesis under both limiting and sat- mating

light conditions. These fndings raised the possibility that the overexpression of FNRmight enhance photosyn- thesis and ultimately lead to increased biomass production. To

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investigate if the accumulation of FNR 'over wild- type levels could improve photosyn-

thetic efficiency and growth, Rodriguez et al. (pp. 639-649) generated transgenic tobacco

(Ncotiana tabacum) plants ex- pressing a pea (Pisum sativum) FNR targeted to

chloroplasts. Contrary to the authors' hopes, however, transform- ants grown at low or

high irradiation exhibited phenotypes and photosynthetic activities comparable to those

of wild type, even though FNR occurred at

levels up to 6-fold greater than wild- type levels. In isolated thylakoids, rates of NADP+photoreduction were corre- lated with FNR levels only when elec- tron donors of PSI

were employed to drive the reaction. The results suggest thatphotosynthetic electron

transport has several rate-limiting steps, with FNR catalyzing just one of them. Not a11

the results were disappointing. In- terestingly, the transgenic Iines with enhanced FNR

exhibited greater toler- ance to photooxidative damage and redox-cycling herbicides that

produce reactive oxygen speeies.

Target Proteins of 14-3-3 Isoforms

Phosphorylation-dependent protein- protein interactions play crueial roles in the

regulation of many biological func- tions. 14-3-3 proteins, which are present in all

eukaryotes, have the ability to bind a multitude of functionally diverse sig- naling

proteins, including kinases, phos- phatases, and transmembrane receptors. There are 15

isoforms in Arabidopsis (Ambidopsis thaliana) and eight isoforms in rice (Oryza satva). 14-

3-3 proteins regulate a variety of different ce11ular processes, such as cell division.

apopto- sis. signaling, and carbon and nitrogen metabolismo Most of the 14-3-3 targets

identified to date.in plants are metabolic enzymes. These results contrast with studies

from animal cells where 14-3-3 proteins are more often involved in signal

perception(receptors), transduc- tion (kinases), and processing (transcrip-

tion factors). This difference is surprising given the conserved nature of 14-3-3proteins.Schoonheim et al. (pp, 670- 683) hypothesize that current views oftheplant 14-

3-3 interactome maybebiased due to the methods used so far to identify 14-3-3 target

proteins. There- fore,they carried out a comprehensive identifica tion of 14-3-3 targets

present in barley iHordeum vulgare) leaf tis sue using two complementary methods: a

yeast (Saccharomyces cerevsae) two- hybrid screen and an affinitypurifica- tion strategy

using a11 five known barley 14-3-3 proteins. Using yeast two-hybrid screens, they

succeeded in identifying 132 proteins that interact with at least one of the fivebarley 14-

3-3 isoforms. The affinity chromatography approach yielded 30 targetproteins with the

ma- jority having a function in primary me- tabolism, possibly reflecting a bias of this

method in identifying more abun- dant proteins. Most of the proteins iden- tified in the

two-hybrid screenare signal mediators,providing evidence thatplant 14-3-3proteins notonlyplay an impor- tant role iri regulation ofthe Calvin cycle, glycolysis, and nitrogen

metabo- lism, but are also important intermedi- ates in signaling cascades inplants.

Extracellular Defense "ProteinsReleased from Root Tips

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The newly synthesized tissue in the region of elongation just behind the root tip of

plants is the primary si te where infectionby nernatodes, fungi, andbacteria is initiated.Yet, surpris- ingly, the apical region encompassing the root cap and root me ristem inplants grown in soil, hydroponics, or laboratory conditions is largely resis-tant to microbial infection. For exam- pie, whenpea roots are inoculated with thepathogenic fungus Nectria haemaio- coeea, most newly generated root tips remain

uninfected even though most roots develop lesionsjust behind the tip in the regionof elongation. The resis- tance mechanism is unknownbut is correlated spatiallywith the presence of border cells on the cap periphery. Previously, it was foundthat more than 100 extracellularproteins are released duringborder cell separation.Wen et al. (pp. 773-783) report that when this root cap secretome is proteolyticallyde- graded during inoculation ofpea roots with N. haematococca, thepercentage ofinfected root tips increases from 4% to 100%. In control experiments, proteasetreatment of conidia or roots had no effect on growth and development of thefungus orthe plant. In addition to defense-related and signalling en- zymes knowntobe present in theplant apoplast, the researchers discovered ri-bosomalproteins,14-3-3proteins, and others typically associated with intra- cellularlocalizationbut

recently shown to be extracellular components of mi- crobial biofilms. Theseresults suggest that the root cap, long known (O release a high molecularweightpolysaccharide mucilage and thousands of living cells into the incipientrhizosphere, also se- cretes a complex mixture ofproteins that appearto functionin protection ofthe root tip from infection.

Transcriptional Profiling oftheArabidopsisEmbryo

Embryogenesis in Arabidopsis is a continuous process, although for conve- nience

it can be separated into three majorphases, described as the globular,heart, andtorpedo stages. The globular s tage is a period af pattem formation andmorphogenesis, during which the axes of the plant body plan are defined andorgan systems formed. The heart stage is aperiod ofmaturation, with a character-istic accumulation of storage reserves. In the torpedo stage, the embryopreparesfor developmental arrest. Spencer et al. (pp. 924-940) have used laser-capturemicrodissection to isolate RN A from discrete tissues of globular-. heart-, andtorpedo-stage embryos of Arabidopsis. This RNA was amplified, and analyzedbyDNA microarrays. Spatial differences in gene expression were four"i to be lesssignificant than temporal differences. Time-course analyses revealed the dy-namics and complexity ofgene expres- sion in both apical and basal domains ofthe developing embryo, and several classes of synexpressed genes were iden-tifiable. The transition from globularto heart stage is associated in particularwithan up-regulation of genes involved in cell cycle control, transcriptional reg-ulation, and energetics and metabolismo The transition from heart to torpedo stageis associated with a repression ofcell cycle genes and an up-regulation ofgenesencoding storageproteins andpathways ofcell growth, energy, and metabolismoThe torpedo-stage embryo shows strong functional differentiation in the root andcotyledon, as inferred from the classes of genes e:'Jressed in these tissues. Theauthors propose that these identified genes can be used to generate cell type-specific markers and promoteractivities for future applica- tions in cellbiology.

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BlockingTranspiration througb a Single Stomatal Pore

Previous microscopic observations of intact elderberry (Sambucus nigra) leavesrevealed that stomata spaced 2 mm apart oscillated independently. This suggeststhat the mechanisms involved in these oscillatory feedbackmechanisms occurat aspatial scale smaller than 2 mm. Thermal imaging and imaging of chlorophyll

fluores- cence. which is taken as a measure for CO2supply to the leaf and corre-sponds to stomatal opening, suggest that stomata inside an intercellularair spacemay behave similarly. These findings indica te that there may be a sensingmechanism that coordinates neighboring stomata. To bettereluci date the spatialscale involved in transpirational sensing, Kaiser and Legner (pp, 1068-1077)evaluated the importance ofa singlepore's tran- spiration on its own response andthat ofadjacent pores. They employed a new method ofapplyng small amountsofmineral oil, which are then car- ried by capillary force into the pores where theyblock transpiration locally. Selected stomata on attached intact elderberry leaveswere sealed with min- eral oil, and the response to a reduc- tion ofhumidity wascontinuously observed in situo The response of sto- mata to a reduction in airhumidity usually consists of hydropassive open- ing followed byactive closure.Theblocking ofa singlepore's transpiration by mineral oil, however, had no ap-preciable effect on hydropassive open- ing and subsequent stomatal closure. Iftheadjacent stomata were additionally sealed, this reduced the closing re- sponsebutnot the hydropassive open- ing. On the other hand,sealing ofthe entire leaf surfaceexcept a small area including the observed stomata also reduced stomatal closure.Contrary to many current hypotheses, these results indicate that strictly localprocesses triggered by a pore's own transpiration are not required to inducestomatal closure.

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